US 7213314 B2
A SAW filter includes a piezoelectric substrate of Lithium Niobate or optionally Lithium Tantalate having a thickness of at least twice an acoustic wavelength. The piezoelectric substrate is bonded to a surrogate substrate of a silicon material. The surrogate substrate is characterized by a resisitivity of at least 100 ohm-cm and an expansion coefficient compatible with the piezoelectric substrate. A catalytic bonding film between the piezoelectric substrate and the surrogate substrate is formed from a first catalytic bonding film deposited onto a surface of the piezoelectric substrate and a second catalytic bonding film deposited onto a surface of the surrogate substrate. The piezoelectric substrate is bonded to the surrogate substrate through a compression force sufficient for providing a bonding at a normal temperature.
1. A method of forming a surface acoustic wave (SAW) filter device, the method comprising:
providing a piezoelectric substrate having a thickness at least twice an acoustic wavelength of a SAW;
polishing a surface of the piezoelectric substrate;
depositing a silicon oxide film onto the surface of the piezoelectric substrate;
polishing an exposed film surface of the silicon oxide film deposited on the piezoelectric substrate;
providing a surrogate substrate characterized by an expansion coefficient less than that of the piezoelectric substrate, and a thermal conductivity for facilitating thermal diffusion;
polishing one surface of the surrogate substrate;
depositing a silicon oxide film onto the one surface of the surrogate substrate;
polishing an exposed surface of the silicon oxide film deposited on the surrogate substrate;
while at a normal temperature, compressing the exposed film surfaces of the silicon oxide films against each other while providing a force sufficient for bonding the piezoelectric substrate to the surrogate substrate; and
forming transducers on an exposed surface of the piezoelectric substrate.
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This application is a Divisional application of U.S. application Ser. No. 10/454,797, filed Jun. 4, 2003 now U.S. Pat. No. 7,105,980 which claims the benefit of U.S. Provisional Application No. 60/393,527, filed Jul. 3, 2002, the disclosures of which are herein incorporated by reference in their entirety.
The present invention relates to surface acoustic wave devices useful for applications in wireless communications.
Surface acoustic wave devices are commonly used in wireless communications for meeting system-filtering requirements. Generally, surface acoustic wave (SAW) filters are manufactured by forming interdigital transducers on piezoelectric substrates such as Lithium Niobate, Lithium Tantalate, Quartz, or the like. Lithium Niobate and Lithium Tantalate have strong electromechanical coupling but generally suffer from high temperature drift, whereas Quartz has stable temperature characteristics but poor electromechanical coupling. For example, a 64° Y-cut, X-propagation Lithium Niobate typically exhibits 70–75 ppm/° C. of temperature drift and has a strong coupling of approximately 11%, while a 46° Y-cut in Lithium Tantalate has a temperature coefficient in the range of 40–60 ppm/° C. and coupling of about 9%. Quartz basically exhibits a zero linear temperature coefficient, but it has a very weak coupling coefficient of 0.15%. For RF SAW filters in wireless phone applications, by way of example, a strong coupling is a desirable substrate characteristic that results in a lower insertion loss and facilitates a filter design having a wider fractional bandwidth. As a result, Lithium Tantalate and Lithium Niobate are generally used in the manufacture of RF SAW filters. However, the relatively high temperature drift associated with these substrates typically precludes their use in SAW devices for certain applications that require a very high close-in rejection. As a result, SAW designers are constantly searching for ways to provide a SAW device on a substrate that can provide both strong electromechanical coupling and good temperature characteristics. A substrate with a combination of strong coupling and temperature stability will allow SAW designers to enhance the flexibility of their SAW device designs and extend SAW filter applications for the wireless phone handset, personal communications service, and PCS duplexer, by way of example, which have a very high close-in rejection requirements. Typically, PCS duplexers have been implemented using ceramic filter technology. While SAW filters are significantly smaller than comparable ceramic filters, the temperature drift of SAW devices on high coupling substrates such as Lithium Tantalate and Lithium Niobate make them undesirable for PCS duplexer application. Duplexers are dual band filters, typically connected directly after the antenna, providing the communications system with simultaneous filtering within the receive path and the transmit signal path. By being so very close to the antenna, key requirements for a PCS SAW duplexer are its power handling capability and low insertion loss along with steep skirt selectivity. Additionally, while Lithium Tantalate has a lower temperature coefficient as compared to Lithium Niobate, it is more pyro-sensitive. The pyroelectric effect is a phenomenon in which an electric field is built up due to a thermal change across the crystalline substrate. If not properly controlled, the pyroelectric effect may cause damage to the SAW device during the manufacturing process, thus reducing yield and reliability of the device.
In view of the aforementioned, numerous techniques have been proposed to improve the temperature characteristics of SAW filters fabricated on high coupling Lithium Niobate and Lithium Tantalate. By way of example, K. Yamanouchi et al. “SAW Properties of SiO2/128° Y-X LiNbO3 Structure Fabricated by Magnetron Sputtering Technique”, IEEE Transactions on Sonics and Ultrasonics, vol. SU-31, No. 1, January 1984) have reported on the deposition of silicon dioxide film on top of the piezoelectric substrate of a 128° LiNbO and 36° YXLiTaO3 to improve the temperature characteristics of the SAW device. Yamanouchi showed that a SiO2 film thickness of 0.25 times the wavelength on a 128° LiNbO3 substrate could reduce its temperature coefficient to approximately zero. Parker et al. (“Stability of SAW Controlled Oscillators”, IEEE 1975 Ultrasonics Symposium Proceedings) demonstrated that the temperature coefficient of the SAW device on YZ LiTaO3 can be reduced to about zero when a layer of silicon dioxide with film thickness of approximately half that of the SAW wavelength is deposited on the substrate. However, these methods suffer from a major drawback in which the SAW properties depend upon the thickness of the SiO2 film. So while it is possible to demonstrate extremely good temperature performance, the SAW device is highly dispersive and thus prohibits the use of these techniques in production. Nakamura (“Effect of a Ferroelectric Inversion Layer on the Temperature Characteristics of SH-Type Surface Acoustic Waves on 36° Y-X LiTaO3 Substrates) discloses the formation of a polarized inversion layer on the surface of the 36° YX LiTaO3to effectively change the elastic stiffness constants of the substrate thereby improving its temperature coefficient of delay. Taguchi et al. (U.S. Pat. No. 5,998,907) discloses a number of SAW composite structures combining the large thermal expansion of Lithium Tantalate and Lithium Niobate with that of a low thermal expansion substrate such as glass or silicon to achieve improved temperature characteristics of the composite device. In one embodiment, a 36° LiTaO3 substrate is bonded directly to a low expansion coefficient glass substrate. In yet another embodiment, a composite structure that comprises a thin film insulation layer of silicon dioxide of 1000-Angstrom is deposited on surrogate substrate of Si 36° LiTaO3 is then bonded to the substrate. This structure as disclosed by Taguchi has a major drawback in the performance of the filter passband characteristics. Since the thickness of the silicon dioxide is so very thin, there exists a direct capacitance coupling between the electrode pattern on the surface of the piezoelectric substrate with that of the Si layer, thus degrading the performance of the filter significantly. Thus, while the structure may provide better temperature characteristics, it suffers with a significant degradation in filter passband performance. This would result in higher insertion loss and a narrowing of the passband that renders them unsuitable for mobile applications.
The present invention may be described by way of example with an embodiment including a SAW filter which includes an electrode pattern deposited on a piezoelectric substrate having a strong electromechanical coupling characteristic, which substrate is bonded at near room temperature to a surrogate substrate through an intermediate catalytic layer to provide desirable temperature compensation for the SAW filter. A very low built in stress level is exhibited at the bonding area, thereby reducing wafer breakage during the fabrication process and improving reliability of the SAW device. The piezoelectric substrate may be Lithium Tantalate or alternatively Lithium Niobate, used with a surrogate substrate of an inorganic Silicon (Si) material. The intermediate layer is comprised of a silicon oxide material. The pre-selected thickness of the silicon oxide material layer provides isolation to a capacitance coupling between the electrode pattern and the Silicon substrate, which in use may be attached to a base. Since Si has a very good thermal conductivity, it improves thermal diffusion through the substrate thereby increasing the power handling capability and reducing the pyro-sensitivity of the SAW device. Yet another embodiment of the invention provides a SAW filter comprising an electrode pattern on a piezoelectric substrate that is bonded to a surrogate substrate through an intermediate bonding layer wherein the thickness of the piezoelectric substrate, the thickness of the bonding layer, the bulk resistivity of the surrogate substrate, and the stress level of the bond area may be changed to adjust the temperature coefficient and passband characteristics of the SAW filter.
One embodiment of the present invention may be described for a surface acoustic wave (SAW) device comprising a piezoelectric substrate having a thickness at least twice an acoustic wavelength of a SAW, a surrogate substrate characterized by an expansion coefficient less than that of the piezoelectric substrate, and a thermal conductivity for facilitating thermal diffusion, and a catalytic bonding film securing the piezoelectric substrate to the surrogate substrate. The catalytic bonding film having a thickness of at least one micron provides isolation between the two substrates to effectively reduce the electrode pattern capacitance coupling. The device will have an expansion coefficient less than that of the piezoelectric substrate.
Another embodiment for a surface acoustic wave (SAW) device comprises a piezoelectric substrate having a thickness at least twice an acoustic wavelength of a SAW operable with the filter device, a surrogate substrate characterized by a resistivity of at least 30 ohm-cm, and a thermal conductivity for facilitating thermal diffusion, and a silicon oxide bonding film securing the piezoelectric substrate to the surrogate substrate.
A method aspect of the invention may include forming a surface acoustic wave (SAW) filter device. One method comprises providing a piezoelectric substrate having a thickness at least twice an acoustic wavelength of a SAW, polishing a surface of the piezoelectric substrate, depositing a silicon oxide film onto the surface of the piezoelectric substrate, and polishing an exposed film surface of the silicon oxide film deposited on the piezoelectric substrate. A surrogate substrate is provided that is characterized by an expansion coefficient less than that of the piezoelectric substrate and a thermal conductivity for facilitating thermal diffusion. The method includes polishing one surface of the surrogate substrate, depositing a silicon oxide film onto the one surface of the surrogate substrate, and polishing an exposed surface of the silicon oxide film deposited on the surrogate substrate. While at a normal temperature, the exposed film surfaces of the silicon oxide films are compressed against each other with a force sufficient for bonding the piezoelectric substrate to the surrogate substrate. As desired, transducers are formed on an exposed surface of the piezoelectric substrate.
Embodiments of the present invention are herein presented by way of example by referring to the following detailed description and drawings in which:
The present invention will now be described more fully with reference to the accompanying drawings in which various embodiments of the invention are shown and described. It is to be understood that the invention may be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the invention to those skilled in the art. Like numbers refer to like elements.
One embodiment of the present invention, a SAW filter 10 illustrated with reference to
The surrogate substrate used in the experiment is an inorganic substrate of Silicon (Si). Since the temperature coefficient of expansion of Si is 2.6 ppm/° C. while that of 46° YX LiTaO3 is approximately 16 ppm/° C., the combined temperature effect on the SAW device will be generally within the range of 2.6 to 16 ppm/° C., depending upon the thickness of the piezoelectric substrate and the stress level at the bond surface. The effective lowering of the temperature coefficient of expansion of the combined substrate structure results in a reduced temperature coefficient of the SAW device. Thus, it is desirable to make the thickness of the piezoelectric substrate much thinner than that of the surrogate substrate. Assuming a sufficient thickness of the surrogate substrate, the thinner the piezoelectric substrate would imply a better temperature coefficient of the SAW device. However, to maintain certain SAW properties of the piezoelectric substrate, particularly the strong electromechanical coupling, it is preferable that the thickness of the piezoelectric substrate be substantially greater than the acoustic wavelength of the SAW device. By way of example, a minimum of 2.5 wavelength thickness limitation on the thickness of the piezoelectric substrate will ensure that predominantly all SAW energy will be confined within the SAW piezoelectric substrate, thereby minimizing the dispersion of the SAW velocity.
For wireless phone applications, SAW filters that operate in the 1900 MHz frequency band would imply a minimum thickness of 5 microns, by way of example. Typically, the thickness of the piezoelectric substrate is set at about 25 microns to facilitate process handling but may be as high as 100 μ (microns). A catalytic bonding layer of silicon oxide facilitates the bonding of the piezoelectric substrate to the surrogate substrate without the use of excessive applied pressure, voltage or extreme elevated temperature. The thicknesses of the bonding layer of silicon oxide may typically vary from 1–15 μ. Such thicknesses are a factor of at least ten times more than that typically used or suggested in the industry as illustrated with reference to U.S. Pat. No. 5,998,907 to Taguchi.
While results of the experiment showed a clear improvement of reduced temperature drift over the uncompensated device, it is useful to compare the passband characteristics of the composite filter with that of an uncompensated filter that will herein be described as a “control unit”. A number of experiments were performed in which the thicknesses of the catalytic bonding layer of silicon oxide and the bulk resistivity of the Si substrate were varied. It was discovered that the silicon oxide not only provides a catalytic bonding layer but also provides an isolation layer between the piezoelectric substrate and that of the Silicon surrogate substrate. With reference again to
By way of example, consider one method that may include forming the surface acoustic wave (SAW) filter 10 earlier described with reference to
Bonding of the piezoelectric substrate 16 to the Si substrate 18 may be accomplished in a number of ways, One technique is described in Patent Application Publication No. US 2002/0064906 A1 (dated May 30, 2002) by Paul Enquist, by way of example. This technique allows substrate bonding at normal temperatures and may be herein applied to the SAW composite device above described. Generally the LiTaO3 substrate 16 is available in the range of 250–500μ thickness. Thus, it is first subjected to a lapping process in which a relatively large amount of substrate is removed by mechanical polishing using slurries with relatively large grinding particles. The lapped piezoelectric substrate is prepared to a thickness greater than 2.5 the wavelength of the surface acoustic wave. Typically, a thickness of about 25μ is selected to facilitate other fabrication processes. A film of silicon oxide is deposited on one of the surfaces of the piezoelectric substrate. The deposition of the silicon oxide onto the piezoelectric substrate may include a chemical vapor deposition, a sputtering or an evaporation process. The layer of silicon oxide is then finely polished.
For the preparation of the surrogate substrate, a surrogate substrate of Si of high resistivity (herein greater than 30 ohm-cm) with thickness typically in the range of 100–500 μ is used. Si is a desirable surrogate substrate because of its low expansion coefficient and high thermal conductivity. The top surface of the Si is chemically polished to a desired smoothness and planarity. A catalytic bonding film of silicon oxide is then deposited through thermal oxidation, chemical vapor deposition, sputtering or evaporation on its surface, by way of example. The silicon oxide is then polished to a very fine smooth and planar surface. The surrogate and the piezoelectric substrates are held together such that the silicon oxide layers of each substrate are in extremely close contact with each other. Once in contact, the substrates are held together as a result of a Van der Waal atomic force. The Van der Waal force is a force that exists between molecules of the same substance. A Van der Waal bond is formed, fusing the two substrates. The strength of this bond can be further enhanced by bringing the bonded substrates to an elevated temperature, while still remaining within a normal temperature range well below typical elevated bonding temperatures. By way of example, the range of normal-temperatures within the teachings of the present invention include 15° C. to 125° C. as opposed to the typical bonding temperature range of 300° C. to 400° C. known in the industry. Since the bonding of the substrates is within this normal temperature range, there is very little built in stress at the bonding boundary surfaces. As a result, wafer breakage during otherwise standard fabrication processes of these wafers is dramatically and desirably reduced. Since Lithium Tantalate is a pyro-electric substrate, a near room temperature direct bonding process produces a robust bond to the surrogate substrate. Additionally, the resultant bonded wafer will be more robust and is less likely to fracture or be damaged during the fabrication process. Thus, this technique also improves the reliability of the SAW devices.
Since bonding stress is one of the factors that control the amount of temperature compensation, it is desirable to select a bonding technique such that one can have good control of bonding stress. Once a composite substrate is accomplished, the fabrication of the SAW device may be accomplished through standard processes. While there are numerous techniques for the fabrication of electrodes on SAW devices, chemical etch or lift off processes are two most commonly used techniques. The chemical etch process of SAW device fabrication would involve the deposition of a metal layer, coating of photoresist, exposure and development of photoresist, chemical etching (either wet or dry etch) and cleaning. The lift off process generally requires a deposition and defining of an electrode pattern of the photoresist, exposure and development of the photoresist, with a metal deposition and lift off the unwanted metal layer. Thus, metal electrode pattern forming on a composite substrate is referred to as a composite SAW device.
By way of yet further example, one requirement for a SAW duplexer is its power handling capability. The power handling capability or durability of the SAW device generally depends upon electron-migration in the interdigitated electrodes and the pyro-electric effect of the substrate. Both these phenomena will depend upon the temperature gradient developed across the SAW substrate. In keeping with the teaching of the present invention, the surrogate substrate 18 may be made of an inorganic substrate of Silicon (Si). Si has high thermal conductivity that facilitates thermal diffusion from the SAW substrate to the package, thereby enhancing power-handling capability and reducing the sensitivity of any pyro-electric effect.
The embodiments above described, by way of example, are SAW filters which may comprise an electrode pattern deposited on Lithium Tantalate which is directly bonded to the surrogated substrate such as Silicon, providing temperature compensation as well as enhancing the power handling capability and reducing the sensitivity of the pyro-electricity of the SAW device. As discussed, Lithium Niobate may also be used as the piezoelectric substrate to be bonded with the surrogate substrate to provide a temperature compensated SAW composite device which has enhanced power handling capability and reduced pyro-electricity sensitivity. Pyro-free Lithium Tantalate may be used to further reduce pyro-electric sensitivity.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and alternate embodiments are intended to be included within the scope of the appended claims.